The present invention relates generally to methods and apparatus for seismic exploration; and more specifically relates to improved methods and apparatus for the initiation of seismic sources during such exploration. The present invention is applicable to all land based sources but offers particular advantages when applied to initiation of vehicle mounted surface sources via radio-controlled signals such as vibratory sources.
The basic principal of seismic surveying, whether on land or at sea, is to periodically actuate man-made sources to produce seismic energy that propagates into the earth. This propagating seismic energy is partially reflected, refracted, diffracted and otherwise affected by one or more geologic structures within the earth. The affected seismic energy is detected by sensors, or “receivers,” placed at or near the earth's surface or the air-sea interface. The receivers are attached to recording instruments that make a permanent record of the detected seismic signals. From the data of these records, the geologic structures of the earth may be analyzed for any of a variety of purposes.
In conventional practice, seismic exploration uses multiple physical sources at each acquisition source location. Such a multiplicity of sources is called a source array and the individual source elements of the array may be distributed both vertically and horizontally in space along or near the earth's surface or the air-sea interface. Typically, the combined energy of the multiple sources is treated as a single source point and the energy generated will be generally concentrated in a particular direction and will form a “beam.”
In the marine environment, complex triggering sequences are normally employed to produce a shaped and directed source pulse from a multiplicity of airguns towed behind the seismic survey vessel. Source-to-source initiation delays are planned in advance through analytical design and experimentation to generate an overall source pulse that will be directed downwardly, through the water column and the earth below the sea floor.
In current practice on land, actively directing the source beam in a specific, selected direction is not commonly done. The use of multiple sources is generally necessary to produce sufficient energy to illuminate structures deep in the earth in situations where a single source does not generate enough energy for reliable detection at the receivers and to structure the source energy pattern to reduce noise and ameliorate the effects of unwanted coherent signal. A recording system connected to multiple physically separated receiver units detects and makes a permanent record of the energy arriving at the receivers, some of which has propagated through the earth's interior.
On land, source initiation sequences have occasionally been employed to similarly shape the source energy produced by an array of multiple dynamite charges. The basic objectives and the analysis and design principles are the same as mentioned above for marine sources.
The dominant source employed on land, however, is the vibratory source. In conventional systems using vibratory sources, the multiple sources are simultaneously initiated by a triggering mechanism that is connected to all the sources and the recording system. The mechanism also provides a “time zero” reference point (the instant of source initiation) for the recording system.
Vibratory sources used on land are limited to placement on the earth's surface. This limitation combined with the current practice of simultaneous source initiation, results in the pointing direction of the resulting source energy beam being perpendicular to the surface. It should be noted that this direction will only be vertically downward where the earth's surface is horizontal. In general the beam pointing direction is vertically downward and not within the control of the survey operator. This can have significant deleterious effects on returning energy levels at the detectors for certain configurations of subsurface reflecting interfaces.
Beam forming tests using vibratory sources have been performed in the past. In currently known examples, such field tests were conducted as series of field tests using source-to-source trigger initiation time delays across a linearly spaced vibrator array. The main purpose of the study was to compare beam steering in the field versus beam steering during data processing. The work was done in an area with a generally planar surface and relatively horizontal subsurface geologic interfaces.
However, when such subsurface interfaces are not generally parallel to the surface, but are oriented in such a way as to reflect the main energy of the source beam in a direction away from the detecting sensors, little useful recorded reflection energy from the interfaces may result. Source energy propagation will be such that certain directions will be more highly illuminated than other directions and the relative strength of detected signals at the receivers will be directionally dependent. Even with horizontal subsurface interfaces, a non-horizontal earth's surface will point the beam in such a manner as to reduce the amount of reflection energy recorded at the detectors. The ability to overcome these deficiencies by directing the source beam in a controlled fashion would be a useful improvement over currently-known methods.
A second deleterious effect results from the placement of sources of any type on the earth' surface. This can result in the generation of unwanted horizontally traveling surface waves, often referred to as “ground roll.” These surface waves are characterized by low frequency content and low velocity, and often arrive at the detectors at the same time as reflection energy from the deeper parts of the earth's interior. When this simultaneous, or near simultaneous, arrival occurs, it often masks the desired deep reflection arrivals needed to make quality images of subsurface formations.
Accordingly, a number of strategies have been adopted by the industry to reduce or mitigate the affects of ground roll. For example, receiver arrays can be designed and deployed to reduce the effects of ground roll at the recording location. Receivers in the array may be spaced and deployed along directions that produce destructive interference of ground roll when the detected signals from the individual receivers in the array are summed together.
An analogous principle has been applied to seismic sources. The spacing and deployment direction of the sources in the source array can be established to suppress the generation of ground roll via destructive interference. This approach assumes the sources in the array are actuated simultaneously and that their spatial organization be accurately designed and deployed; as the effective implementation of this technique relies upon the placement of sources according to a relatively exact spacing. Any deviation from this ideal placement will result in degradation of the suppression of ground roll effects. An exception to this simultaneous initiation of vibratory sources is a method of U.S. Pat. No. 7,050,356, where non-simultaneous source initiation is used for reducing survey time. This approach still suffers from the problems and effects from source placement errors, and the ground roll suppression inefficiencies that result.
In conventional source array design, the surface is typically assumed to be a planar, horizontal surface, along which the sources will be distributed. The ideal horizontal source separation distance is directly related to the dominant frequency and propagation velocity of the ground roll to preferably be suppressed. If the sources all occupy the same plane, that is, there are no vertical separations, the combined source energy or beam will be directed vertically downward. The simultaneous triggering of the multiple sources forces this situation; and is advantageous when the objectives of the data acquisition lie vertically below the source and receiver arrays and are configured as horizontal or gently dipping interfaces. However, this vertical direction of the beam can lead to less than optimal results where the subsurface formations are not parallel to the surface.
Another possible source of error in land-based seismic exploration systems is that of inaccurate placement of the sources forming the ground roll suppressing array. Current practice does not generally provide for surveying each individual vibratory source location in advance (currently a costly and time-consuming process), to provide sufficient information to assure the ability to place each vibratory source correctly. This problem can be further complicated by unavoidable obstructions and other surface conditions that prevent the placement of one or more sources of the array in the intended location. Such source placement errors limit the ability of the source array to optimally suppress ground roll. Additionally, in many instances, the configuration of the earth being seismically surveyed will not lend itself to such exact placements. For example, there are often elevation variations of the surface that violate the basic assumptions of the array design.
The present methods described herein combine the ability to correctly position the elements of the source array, and to achieve a desired beam direction while also allowing suppression of ground roll. Accordingly, the present invention provides a new method and apparatus which uses controlled placement and initiation of seismic sources to improve the performance of seismic exploration systems and operations. The invention permits directing the source energy beam while at the same time allowing ground roll suppressing source arrays to be optimally deployed. The present invention offers additional advantages and benefits as will be set forth herein.